In recent years, fabrication of semiconductor lasers has been studied using silicon materials, which are unsuitable by nature for light sources due to their low light emission efficiency.
For example, evanescent lasers have been developed, wherein a group III-V material with a higher light emission efficiency is wafer-bonded over a silicon waveguide formed over a silicon substrate, such that light propagating through the silicon waveguide evanescently couples to the group III-V material, thereby providing an optical gain. In other words, hybrid integrated evanescent lasers have been developed, wherein a group III-V material is hybrid integrated to a silicon substrate.
Some of such hybrid integrated evanescent lasers have a distributed feed-back (DFB) laser configuration, wherein a diffraction grating with a λ/4 phase shift is provided on the surface on the silicon substrate side, thereby attaining single mode oscillation. Another type of lasers has tapered both end faces of a group III-V material in order to reduce reflections at the both end faces of the group III-V material.